I'd say "The Menagerie" is probably the most overrated episode. Seeing the first pilot adds a nice bit of back story, and Spock's decision to violate regulations in Part I is a strong, dramatic hook, but Part II is a dud where the characters spend most of the episode watching TV punctuated by contrived cliffhangers at the act breaks which are all too easily resolved.
And yet, it's a Hugo winner. Season one had a number of episodes which were much, much stronger.
Oh, we're gonna get offtrack...
I find the ending from the point Mendez disappears to be utterly fantastic. And a perfect illustration of how early Shatner wasn't just good, he was fantastic at times.